Changes of watershed management against floods in international rivers

摘要

The historical and future directions of water management against floods are discussed and the functions of paddy areas evaluated in terms of flood retention and mitigation in neighboring urban areas. Several examples of several rivers and watershed management against floods are discussed, such as the Tone River (the largest basin area in Japan), the Rhine River in Europe, the Mississippi River in the United States, and the Mekong River in Asia. In the past, agricultural lands are regarded as having only the role of food production, but now agricultural areas mainly composed of paddies are considered to have several functions, such as flood protection, fostering of water resources, prevention of soil erosion and so on. Hence, in order to solve various problems of water resources including floods, a method of evaluating the multi-functional roles of paddies is proposed, covering the two aspects of 1) usage of multi-functional roles as basin-wide management, and 2) technical problems to be solved. The results obtained emphasize the importance of evaluating the multi-functional roles of agricultural lands, the perspectives of flood control and floodplain management in the 21st century and a method to evaluate function from a macro point of view.